They may wish to pursue both but the issue remains of finance: if you're paying out lots of Francs to have babies, thats Francs that don't go to funding colonial expeditions.
The problem is that you don't understand what sort of proposals the French had for raising the birth rate: none of them were large government intervention programs. The only thing I can think of among the list that was introduced was the family allocation schemes, but its fully possible to have a pro-natalist scheme without that, and even if it was introduced, its relatively easily supportable given that the French social welfare system was not very costly in the first place. The 1896 proposals and those in the years following when the issue was first seriously brought up included that, but the rest was all cheap and favoritism schemes or punitive: prizes, school benefits, military service reductions for fathers, taxes on bachelors, faster promotion for bureaucrats with more children, maternity leave, some discussion about changing inheritance laws, etc. Restrictions on abortion and contraceptives fall into the same manner.
The modern French pro-natalism is the product of a society which is markedly different from the 1870 one and which has far larger tools for state intervention. The 1870 proposals wouldn't be seeing the great expenses that the current French state provides, but instead as part of a effort to regenerate and revitalize society along conservative lines, which is something which will for the most part be done on the cheap, since it will be viewed as a moral struggle first and foremost, and not one which requires the state itself to be the principal actor.
Which was mostly the case in 19th Century life anyway and we've had plenty of examples of trying this same thing in the 20th and 21st century; impacts were non-existent at worst and negligible for a duration at best.
You're both not reading my post as I state that later on, and not understanding it: the focus is not whether these would work, but what sort of proposals would be implemented. These were all the sort of proposals that were later on discussed or introduced in the possible liberal framework of the IIIrd Republic.
Interesting you mention Morocco as it's Lyautey's turf.
One thing of his I remember reading is that France could get new, fresher blood through colonisations after 1870.
Now you can read this in 3 ways:
Like you did, with French settlers able to have bigger families in New lands
As an incentive to get more immigrants like Spanish ones in Algeria
Or as an advice to actively mingle with the subjects to get higher numbers
I always read it as the last one, but I'm not sure he's very representative of the colonial establishment
Generally the idea was as you said not focused on mingling with the natives, the French pro-natalists who were interested in colonial settlement had some evidence, I don't know if it was real or not but they believed it, that French settlers in overseas territories like Algeria had much higher birth rates than back at home. So their idea was to focus on aiding colonial settlement, which was a dramatic reversal of previous beliefs which held that emigration overseas would depopulate France. It has been a while since I have read A Colonial Fountain of Youth: Imperialism and France's Crisis of Depopulation, 1870-1940 unfortunately, so I would have to re-read it again to provide additional detail.